The present invention relates to fish fence primarily of an immaterial type, based on subsea use of low frequency mechanical (infra acoustic) vibrations in conjunction with an electric field where two such signals are modulated synchronously. The infraacoustic field combined with the electrical field create anxiety reactions in fish. Fish also will feel pain caused by the electric field.
Usually, a fish fence is material, i.e. sometimes a net is used acting as an obstruction for fish exceeding a certain size.
However, in certain cases such a material fence constitutes a problem, e.g. by being a hindrance to other sea traffic, by the fact that the fence is exposed to fouling, as well as by the fact that it is difficult to alter the mesh in order to vary the size of the fish it is desirable to have pass through the fence. Consequently, a different type of fence is needed, for example an immaterial "energy fence", also called "non-physical" fence, of a type to irritate/frighten and cause pain, so as to make fish turn back when about to swim into the fence.
Previously there have been conducted experiments using different forms of both sound and electrical fences, however not in combination. In the majority of experiments where sound has been used, sound pressure has been used, in the sense that the actual sound pressure is used to frighten away the fish or to make the fish stay behind or inside an imaginary barrier.
These experiments to generate sound pressure in water have all been conducted within the hearing range of the fish (i.e. the range 50 Hz-approximately 2000 Hz), audible to fish via otolith and/or swimming bladder, but these methods have proved to be not very effective, for the simple reason that the fish gets used to the sound.
Additionally, several experiments have been performed with electrical fences with an aim at establishing an electrical field in the water. Experiments have been conducted with different frequencies and pulse widths in an alternating electric field and, generally, it can be stated that extremely varied results were achieved. The best results were obtained when using a very short pulse width and frequencies of 50 Hz. In this case a barrier efficiency of approximately 80% was achieved, but small fish were nevertheless able to swim rather easily through the fence, and larger fish were killed. The reason for these problems is that the fish have no organs to indicate from which direction the electric field comes, this resulting in that the fish, upon entering the electric field, will feel pain, but will not be able to detect the reason therefor. Usually it will continue swimming into the field, where it will either be killed or may succeed in passing through (sometimes injured), depending on its physical size.
New investigations show that fish exposed to infraacoustic particle movements (both the acceleration and velocity of the water particles are important) can detect such movements almost down to a frequency of 1 Hz (experiments have been performed down to 3 Hz) with the use of side line organs. In addition to detecting the particle movement (the particle acceleration), the fish is also able to detect the direction.
Fish have such a sensor system as a predator warning, and experiments show that fish exposed to infra acoustic accelerations get spontaneous reactions of fear (under 20 Hz).
Experiments with an infraacoustic fence with a dipole type acceleration pattern can produce relatively good results, but the result is nevertheless too dependant on the surroundings, i.e. factors like stress level (traffic), conditions of food/light and threshold of fear (due to possible pedators in the surroundings).
From U.S. Pat. No. 2,146,105 is previously known electrodes which create electrical dipole fields, but no additional stimulus of the acoustic type to impart fear and direction information to the fish, appears therein.
A combined system, however, is known from U.S. Pat. No. 2,709,984but this combination relates to light signals and electric influence of the fish from a plurality of electrodes suspended in the water. Correctly, it is stated in such publication that the fish needs a "direction" indicator in addition to the electric shocks sensed, in order to realize in which direction it must swim to avoid the unpleasant effect. However, important differences exist between light and sound regarding such direction indication, as light is a positive stimulus and hence the fish is dependent on training to achieve the desired effect (i.e. the fish must collate a flash of light with a simultaneous electric shock).
However, it would be far more advantageous if an immediate response for the fish could be obtained without any prior learning process.